The present invention relates to new and improved circuits for decoding electrical pulse signals.
It is sometimes desirable to encode an electrical signal for transmission over wire, optical or radio link so that it can be separated from noise or other spurious signals that may be present. Applications include enabling, arming or firing a weapon system in which such conditions exist. For example, consider a complex missile system where frequent electrical testing may be expected while the weapon is in stockpile. The test signals must not initiate the warhead arming cycle. In addition, the warhead arming must be delayed after launch to provide the maximum protection to friendly personnel and territory. During this portion of flight, electrical currents and other signals may be present in the missile circuit performing warhead functions. The missile arming and fuzing system must be designed to protect against these stimuli affecting the warhead sub-system. Numerous methods of accomplishing this purpose have been developed and used. One method is to have an arming function dependent upon a "unique" signal that is neither a direct current nor a continuous alternating current. Such a signal should present some characteristic that would prevent confusion with a spurious signal.
A suitable unique signal is produced by modifying a pulse oscillator that is widely used in digital electronics and consists of three phase inverters connected in series, with a timing capacitor connected in parallel with the second inverter and a low-impedance feedback connection between the output of the third inverter and the input of the first inverter. This oscillator generates a pulse signal train consisting of pulses and spaces between pulses of uniform length (with times) and no identifiable starting time. The modification involves the substitution, for the third inverter, of either a NOR gate or a NAND gate having two inputs, one of which is connected to the output of the second inverter, and means for applying a control or gating signal to the other input. The modified oscillator produces a pulse train made up of a first pulse of given length followed by spaces and pulses of much shorter length, the unique feature being the long first pulse.
The invention involves the provision of a new and improved decoding circuit including the new pulse oscillator, for generating a unique pulse train starting with a long first pulse and followed by short spaces and pulses, in combination with means for comparing the locally generated pulse train with a similar, but not necessarily identical, input signal pulse train, and either rejecting a spurious signal or accepting the signal and producing an output, for any purpose desired. The local oscillator is started by the first pulse of the incoming signal train, which could be the beginning of a true or desired signal. The decoding circuit further includes: two digital counters, one connected to the pulse oscillator and the other connected to the signal source; and either an AND gate or a NOR gate having two inputs connected, respectively, to the first counting outputs of the two counters in phase opposition, a third input connected to the oscillator, and an output connected to means for resetting the oscillator and the two counters in the event of a mismatch between the first pulses of the oscillator and signal pulse trains. The decoding circuit further includes a second inverting gate, which may be either an AND gate or a NOR gate, having two inputs connected, respectively, to the Nth counting outputs of the two counters in phase opposition, a third input connected to the oscillator, and an output connected to the resetting means, for rejecting the signal in the event of a mismatch between the Nth pulses of the signal and oscillator trains, and an output gate having two inputs connected, respectively, to the Nth counting outputs of the two counters in the same phase relationship, a third input connected to the oscillator, and an output connected to an output terminal, for accepting a matching signal and producing an output signal, which may, for example, be used to initiate an arming circuit for a munition fuze. In another example, the decoding circuit may be a part of a lock for a high security area, and the signal may be the key for opening the lock.